I think it could be quite good option in filter box. It will be great if could be possible to select soundtracks where specific composer is responsible only for BGM´s.

It's not possible with the current system. We would have to add new kinds of classifications, perhaps similar to the ones seen on product pages (soundtrack, op, ed, insert, etc.), and then manually assign them on every album.

Blah has some good ideas for how to deal with this kind of thing (in general), but it's a massive job. Maybe one day.